The present invention relates to a method for operating a direct current metal halide arc lamp, to an associated circuit arrangement, and to a direct current metal halide arc lamp with a fill that is especially well suited to these purposes.
For example, direct current metal halide arc lamps are needed for projection applications. For good color reproduction, the spectrum at the location of the highest luminance, that is, upstream of the cathode, should include sufficient proportions of the primary colors, i.e., blue, green and red. It is known to use the fill elements of indium for blue and lithium for red. In typical projection lamps, however, the primary color red is especially lacking, since the radiation of the element lithium is emitted predominantly not from the site of highest luminance but from the jacket of the arc instead. It is true that the proportion of red in the light generated can be enhanced by increasing the proportion of lithium, but then it must be remembered that lithium predominantly has very long-wave emissions, thus producing a very dark red component. Since the spectral sensitivity of the human eye drops off markedly at the long-wave edge, then to the extent that the red component is based on the lithium emissions, a correspondingly enhanced spectral power must be generated if the desired light flux is to be generated. On the other hand, it has been found that adding lithium to the lamp fill increases the so-called color separation effect; that is, various spectral ranges of the light generated are generated at different sites in the lamp; this worsens the light quality for projection purposes, which is expressed in color fringes at the boundary or peripheral regions of projected images.
Corresponding problems arise in the operation of rectangular alternating current lamps.
For generating a discharge with enhanced brightness, it is known from German Patent Disclosure DE 39 20 675 to operate a short-arc discharge lamp with a constant base current, on which a periodic pulsed current is imposed. The pulse length is in the range from 0.03 to 3 ms, and the intervals between pulses vary between 0.1 and 10 ms. Triggering a direct current arc lamp with a signal whose intervals between pulses are in this range would cause the direct current arc lamp to go out, especially if an additional base current of high constant amplitude is not used. No relationship between the trigger signal and the spectrum of the light generated can be learned from this reference.
European Patent Disclosure EP 0 443 795 and U.S. Pat. Nos. 5,047,695 and 5,198,727 describe DC discharges with AC xe2x80x9cripplesxe2x80x9d superimposed on them; the AC ripples are in the frequency range between 20 and 200 kHz for acoustically tightening the arc.
It is therefore the object of the present invention to propose a method for operating a direct current metal halide arc lamp, in particular a direct current metal halide arc lamp for projection purposes, or a rectangular alternating current lamp, by means of which the photometric data are improved. It is also an object of the present invention to describe an associated circuit arrangement, as well as a direct current lamp with a fill that is especially well suited to operation according to the invention.
According to the invention, this object is attained by the characteristics of the independent claims.
The fundamental concept of the invention is to operate the direct current metal halide arc lamp with a clocked voltage signal. The signal is cyclically clocked during a period Tein to an ON amplitude and during the subsequent period Taus to a voltage of quantitatively lesser amplitude.
Advantageously, the time period Tein is between 10 and 100 xcexcs, and the time period Taus is between 1 and 50 xcexcs. The same is true for the operation according to the invention of rectangular alternating current lamps.
The invention offers the advantage of markedly increasing the radiation, upstream of the cathode, of the element lithium, or other elements of group 1A, that is, the red component. Since the normal calibration curve xxcex is at its maximum in this spectral region, the tristimulus value x rises compared to y. Thus by adding an element with radiation lines in the range from 520 to 580 nm, such as thallium at 535.1 nm, the y value can be increased without exceeding the Planckian locus, and without the perceived color shifting toward greenish. Increasing the y value also increases the useful light flux. Surprisingly, in the operation according to the invention of direct current metal halide arc lamps, the change in convective flow conditions in the lam causes a marked reduction in the electrode temperatures, especially for the anode that is usually overloaded in metal halide d.c. lamps. This leads to an improvement in the light flux drop over time, or so-called maintenance, since there is a reduction in blackening and electrode consumption. The result is a longer service life of the direct current arc lamp.
In the circuit arrangement of the invention, it has proved especially advantageous to select the operation of the pulsator such that in the pulsator output signal, the voltage is essentially 0 V during the period Taus. The same is correspondingly true for the circuit arrangement of the invention for operating a rectangular alternating current lamp; that is, in this case the amplitude values are Un and xe2x88x92Un during the time periods Taus and Txe2x80x2aus (see FIG. 4), and advantageously both are essentially 0 V.
To prevent acoustical resonances, the time period Tein or Txe2x80x2ein can be varied periodically, for instance being swept with a sweep frequency of 50 to 500 Hz, preferably 100 Hz. The time period Taus or Txe2x80x2aus can either be constant or be varied as well. If Taus or Txe2x80x2aus is varied, then especially advantageously it is possible to perform a variation with adaptation to the variation of Tein and Txe2x80x2ein, with the goal that the minimal voltage value generated in the signal downstream of the starter for triggering the rectangular alternating current lamp remains quantitatively constant despite the variation of Tein and Txe2x80x2ein, respectively. Other advantageous embodiments are described in the dependent claims.